FIG. 1 shows a data transmission system according to the prior art. Between a transceiver at the switch end LTU (Line Terminating Unit) and a transceiver at the subscriber end NTU (Network Terminating Unit), the signaling data are first transmitted via a data transmission line for setting up a data transmission link for user data. The transceiver at the subscriber end NTU is a connecting socket which is installed in customer or subscriber premises by the network operator. The transceiver at the subscriber end NTU is connected to a data terminal, for example a computer. The transceiver at the switch end LTU is connected to a network, for example the telephone network. As a rule, the data transmission line is a copper telephone pair.
To set up a data transmission link, the two transceivers must be brought from a standby mode into an activated operating mode for the data transmission. In xDSL data transmission methods, the twisted analog telephone pairs are used as wide-band connections in the local loop. The most well-known xDSL data transmission methods are ADSL (Asymmetric DSL), VDSL (Very-High-Data-Rate DSL), SDSL (Symmetrical Single-Pair DSL).
After the completed cold start and subsequent deactivation, the two transceivers are initially in a power-saving or standby mode. In the power-saving mode, the transceivers consume minimum power so that the heat generated is minimum and no cooling is necessary.
FIG. 2 shows a link between two transceivers LTU, NTU according to the prior art in detail. To set up a data link, the transceivers are reactivated in a warm-start sequence.
FIG. 3 shows a possible warm-start sequence according to the prior art, which is described in German Patent Application DE 101 39 779.8.
Initially, one of the two transceivers sends a wake-up signal via the data transmission line to the remote transceiver in order to activate the latter. In the sequence shown in FIG. 3, the transceiver at the subscriber end NTU sends a wake-up signal WWUN to the transceiver at the switch end LTU. After a predetermined interval TWS, the transceiver at the switch end LTU also sends a wake-up signal WWUL back to the modem at the subscriber end NTU via the data transmission line. With this wake-up signal, the modem at the switch end LTU acknowledges reception of the original wake-up signal WWUN.
After the wake-up sequence, it is determined in a line probing phase whether the line parameters of the data transmission line have changed since the last activation. For this purpose, the transceiver at the subscriber end NTU sends a line-probing signal WLPN via the transmission line to the transceiver at the switch end LTU in the example shown in FIG. 3. The transceiver at the switch end evaluates the received signal. After a predetermined waiting time TWS, the transceiver at the switch end LTU conversely also sends a line-probing signal WLPL to the transceiver at the subscriber end NTU. The modem at the subscriber end NTU evaluates the received signal.
After the line parameters have been checked, a complete echo signal elimination may take place in a further phase. Since the last activation of the two transceivers, the line parameters of the data transmission line may have changed. This leads to a residual echo signal of the inherent transmit signal of a transceiver. The residual echo signal may impair the detection of a data signal received from the remote transceiver. The echo signal must, therefore, be canceled by echo signal elimination in such a way that it is below a predetermined threshold value. To eliminate the echo signal, the modem at the subscriber end NTU in the example shown in FIG. 3 sends out a signal for echo cancelation and the remote modem at the switch end LTU does not send out a signal. The echo cancelation circuit in the modem at the subscriber end NTU is set with the aid of the WECN signal. After a predetermined waiting time tWS [sic], the modem at the switch end LTU then also sends out a signal WECL for echo signal cancelation. During this time, the other modem NTU does not send out a signal. The echo cancelation circuit in the transceiver at the switch end LTU is adjusted to minimize the residual echo signal with the aid of the adjustment signal WECL.
After the echo signal elimination, synchronization is effected between the two transceivers in a synchronization sequence. The modem at the subscriber end NTU sends out a synchronization signal WSN for synchronizing the transceiver at the switch end LTU and conversely the modem at the switch end LTU sends out a synchronization signal WSL for synchronizing the modem at the subscriber end NTU. As soon as the two transceivers are synchronized, they in each case send an indicating signal to the other transceiver which indicates the completed synchronization. Following this, user data are transmitted between the two transceivers.
For a warm start between two transceivers, it is desirable that the period for the warm start twarm start is as short as possible. In the warm start sequence according to the prior art, the probing of the data transmission line, the echo signal elimination and the synchronization take place after one another or serially. A serious problem in this is that a predetermined worst-case time is in each case provided for probing the data transmission line and for eliminating the echo signal in the standard protocol for setting up the data transmission link. For this reason, the period for the warm start is relatively long in the warm start sequence according to the prior art.